In capturing an image in a typical camera, light enters through an opening (aperture) at one end of the camera and is directed to a focal plane by an optical array. In most cameras a lens stack including one or more layers of optical elements are placed between the aperture and the focal plane to focus light onto the focal plane. The focal plane consists of light sensitive pixels that generate signals upon receiving light via the optic array. Commonly used light sensitive sensors for use as light sensitive pixels include CCD (charge-coupled device) and CMOS (complementary metal-oxide-semiconductor) sensors.
Filters are often employed in the camera to selectively transmit lights of certain wavelengths onto the light sensitive pixels. In conventional cameras a Bayer filter mosaic is often formed on the light sensitive pixels. The Bayer filter is a color filter array that arranges one of the RGB color filters on each of the color pixels. The Bayer filter pattern includes 50% green filters, 25% red filters and 25% blue filters. Since each pixel generates a signal representing strength of a color component in the light and not the full range of colors, demosaicing is performed to interpolate a set of red, green and blue values for each pixel.
Cameras are subject to various performance constraints. The performance constraints for cameras include, among others, dynamic range, signal to noise (SNR) ratio and low light sensitivity. The dynamic range is defined as the ratio of the maximum possible signal that can be captured by a pixel to the total noise signal. The maximum possible signal in turn is dependent on the strength of the incident illumination and the duration of exposure (e.g., integration time, and shutter width). The signal to noise ratio (SNR) of a captured image is, to a great extent, a measure of image quality. In general, as more light is captured by the pixel, the higher the SNR. Accordingly, the SNR of a captured image is usually related to the light gathering capability of the pixel.
Generally, Bayer filter sensors have low light sensitivity. At low light levels, each pixel's light gathering capability is constrained by the low signal levels incident upon each pixel. In addition, the color filters over the pixel and the necessity to confine the chief ray angle incident on the pixel to avoid cross-talk further constrain the signal reaching the pixel. IR (Infrared) filters also reduce the photo-response from near-IR signals, which can carry valuable information. These performance constraints are greatly magnified in cameras designed for mobile systems due to the nature of design constraints. Pixels for mobile cameras are typically much smaller than the pixels of digital still cameras (DSC). Due to limits in light gathering ability, reduced SNR, limits in the dynamic range, and reduced sensitivity to low light scenes, the cameras in mobile cameras show poor performance.
Quantum dots are semiconductor particles that can take any number of shapes including cubes, spheres, pyramids, etc., and have a size that is comparable to the Bohr radius of the separation of electron and hole (exciton). The electronic characteristics of quantum dots are closely related to the size and shape of the individual crystal. In particular, when the size of the quantum dot is smaller than the exciton Bohr radius, the electrons crowd together leading to the splitting of the original energy levels into smaller ones with smaller gaps between each successive level. Thus, if the size of the quantum dot is small enough that the quantum confinement effects dominate (typically less than 10 nm), the electronic and optical properties change, and the fluorescent wavelength is determined by the particle size. In general, the smaller the size of the quantum dot particle, the larger the band gap, the greater becomes the difference in energy between the highest valence band and the lowest conduction band, therefore more energy is needed to excite the dot, and concurrently, more energy is released when the crystal returns to its resting state. For example, in fluorescent dye applications, this equates to higher frequencies of light emitted after excitation of the dot as the crystal size grows smaller, resulting in a color shift from red to blue in the light emitted. Beneficial to this tuning is that a high level of control over the size of the quantum dot particles produced is possible. As a result, it is possible to have very precise control over the fluorescent properties of quantum dot materials.
Quantum dots can be assembled into light sensitive quantum films that can be integrated onto a silicon wafer at the pixel level to replace the silicon photodiode-parts of a CCD or CMOS sensor. These quantum films are significantly more sensitive to light than are standard CMOS sensors that utilize a silicon photodiode. In addition, because the photosensitive volumes of the quantum film is not deep in the structure, as it is in a CMOS image sensor, but at the light sensitive quantum film surface, there is reduced cross talk between neighboring pixels, which relaxes lens CRA requirements.